Certain types of printing systems are adapted for printing images on large-scale substrates, such as museum displays, billboards, sails, bus boards, and banners. Some of these systems use so-called drop on demand ink jet printing. In these systems, a piezoelectric vibrator applies pressure to an ink reservoir of the print head to force the ink out through the nozzle orifices positioned on the underside of the print heads. A particular image is created by controlling the order at which ink is ejected from the various nozzle orifices.
In some of these systems, a carriage which holds a set of print heads scans across the width of a flexible substrate while the print heads deposit ink as the substrate moves. In another type of system, a solid, non-flexible substrate is supported on a table. The carriage holding the print heads has two degrees of motion so that it is able to move along the length as well as the width of the substrate as the print heads deposit ink onto the substrate. And in yet another arrangement, a solid, non-flexible substrate is held to a table as the entire table and substrate move together s along one axis of the substrate under the print heads as the carriage holding the print heads traverses in a direction normal to that axis while the print heads deposit ink to create a desired image.
To print on solid, non-flexible substrates, operators typically first print on a flexible substrate and then laminate the substrate onto a solid, non-flexible base. As for printing systems that print directly only solid substrates, the size of the substrate upon which the image can be printed is limited. For example, a carriage with two-degrees of motion can only travel to the extent of the physical dimensions of the rails along which the carriage travels. As for systems in which the table along with the substrate moves under the print heads, the substrate can be no larger that the size of the table. It is desirable therefore to be able to print on both flexible and non-flexible substrates with varying thicknesses, and to be able to accommodate substrates with various stiffnesses and thicknesses automatically with little or no intervention from the operator.
In one aspect of the invention, a system for printing images on a substrate includes a multiplicity of print heads mounted in a carriage and positioned a distance from the substrate. A sensor detects the thickness of the substrate as the substrate moves through the system, and a control system receives the substrate thickness information detected by the sensor and transmits signals to a motor coupled to the carriage. These signals instruct the motor to adjust the position of the carriage to maintain a desired gap between the print heads and the substrate. The minimum gap can be about 0.04 inch, and the maximum gap can be about 0.08 inch.
Embodiments of this aspect can include one or more of the following features. The control system includes a controller which transmits the signals to the motor. The controller is coupled to a CPU which receives a substrate thickness information signal from the sensor, processes the information, and transmits signals to the controller to instruct the motor to adjust the position of the carriage to maintain the desired gap. The control system includes a feedback device which senses the gap between the print heads and the substrate. The gap information is relayed to the controller such that the controller can further instruct the motor to alter the position of the print heads relative to the substrate to achieve the desired gap. In certain embodiments, the feedback device transmits the gap information to a CPU which processes the information and relays the processed gap information to the controller. The motor can be a servo motor.
In some embodiments, the position of the carriage is adjusted in less than about five seconds. The sensor can include an indicator roller, and a dial indicator can be coupled to the indicator roller.
A related aspect of the invention includes a method for controlling the distance between print heads of a printing system and a substrate. The method includes detecting the thickness of the substrate, and transmitting the thickness information to a controller. The controller transmits height adjustment information to a motor coupled to a carriage which holds the print heads. The motor then adjusts the position of the carriage to maintain a desired gap between the print heads and the substrate.
The method can include detecting the distance between the substrate and the print heads, and the position of the print heads can be re-adjusted based on the distance information. The gap maintained between the print heads and the substrate can be approximately in the range 0.04 inch to 0.08 inch.
Among other advantages, the printing system of the present invention is capable of printing on both flexible and non-flexible substrates without manually adjusting the gap between the print heads and the substrate.